The cytoplasmic site of gene expression and use of virally encoded enzymes are distinguishing features of vaccinia virus and other poxvirus vectors that contribute to their consistent ability to express genes derived from a variety of prokaryotic, eukaryotic and viral sources. This feature, together with their ability to stably integrate and package large amounts of additional DNA, and broad host range, account for their general use as a research tool and for synthesis of recombinant proteins in mammalian cells. Recombinant vaccinia viruses are also being tested as live vaccines. During the past year, we have been evaluating the highly attenuated modified vaccinia virus Ankara strain as a safe vector in the laboratory and as a vector for candidate vaccines against infectious diseases and cancer.